DESCRIPTION (Investigator's Abstract): The broad goal of this project is to understand the factors which limit spatial vision in normally sighted individuals, and in four degraded visual systems: i) anisometropic amblyopes, ii) strabismic amblyopes, iii) peripheral vision, and iv) young children. Each of these visual systems exhibits particular losses in fine position discrimination. A major goal of this project is concerned with the mechanisms which constrain resolution and positional acuity in these visual systems by comparing them to foveal vision that is degraded by: i) low contrast, ii) positional jitter, iii) sparse sampling, iv) low luminance, and v) camouflage. The investigators will also do parallel studies in amblyopes, in the normal periphery, and in young children, in order to learn about "intrinsic" constraints. The investigators propose to test a number of specific hypotheses and models with carefully chosen psychophysical methods and stimuli. For example, the investigators hypothesize that the Vernier thresholds of anisometropic patients, but not strabismics, will be consistent with their sensitivity to local contrast, whereas Vernier acuity in strabismic, but not anisometropic, amblyopes will be mimicked in the normal fovea by undersampling and jittering the stimulus. The investigators propose two experiments to test the hypothesis that strabismic amblyopes are "cortically undersampled": a) comparing grating detection and orientation discrimination, and b) "motion reversal". The investigators predict that the normal periphery shows a loss of positional acuity which will not be fully accounted for by either local contrast sensitivity, or by "front-end" mechanisms. The investigators' hypothesis is that Vernier acuity in peripheral vision will be mimicked by undersampling the stimulus, and spatial jitter. The investigators also propose to test two recent models which make specific predictions about the vision of young children, based on different constraints. Noise masking will be used to estimate the orientation and spatial frequency selectivity of the putative spatial filters underlying Vernier acuity. The investigators hypothesize that: i) in normal foveal vision, masking will be different for small versus large gaps, reflecting two separate mechanisms; ii) peripheral and amblyopic masking functions will be similar to normal foveal functions, but scaled to lower spatial frequencies. The investigators propose to measure Vernier acuity and resolution across the visual field of amblyopes to test the hypothesis that the amblyopic deficit in positional acuity is locus-specific and restricted to the perifovea. The investigators also propose to map out in detail the variation of resolution and Vernier acuity thresholds within the central 2.5 degrees, under conditions of stabilized retinal imagery, in order to compare psychophysical data to recent anatomical data on the retinal mosaic. Amblyopia is the most frequent cause of visual loss in childhood, and is thus a significant developmental disorder. Thus, a goal of this project is to determine whether the losses found in adults with amblyopia are representative of the abnormalities of children with amblyopia, and to measure the recovery of resolution and position acuity in children undergoing treatment for amblyopia.